Industry standards for AC and high power DC capacitors have traditionally centered around oil filled capacitor technology. This technology offers benefits of high corona resistance and transient capabilities. Capacitors using this type of technology, however, have problems of potential oil rupture, expensive housings and terminals, poor high frequency response (noisy), mounting restrictions and increased weight. Oil fill technology traditionally employs series disconnects which remove the capacitor from the circuit (or system) by physical distortion of the capacitor housing to break the conductor. These capacitors are permanently disconnected from the circuit and cannot be reset.
Dry film technology offers advantages over oil fill technology. These advantages include broad frequency range, low power loss, light weight, and self healing devices without liquid rupture potential or mounting restrictions. Dry film capacitors, however, have a failure mode that is typically not found in oil fill capacitors. This failure mode is caused by the quality of the capacitor and its electrode configuration which does not allow the capacitor to go to a low resistance short. Instead, the capacitor continually self heals, as the operating temperature inside the capacitor is increased above its operational limits. As the healing continues, the capacitor continues to function and becomes hotter. This, in turn, causes further healing and leads to an avalanching affect. Eventually the capacitor goes to a high resistance short of several ohms, which acts similarly to a heater inside the capacitor and leads to thermal runaway and to gas release due to decomposition of its polymer material and electrodes. The onset of these conditions may arise from misapplication of the capacitor, end of life of the capacitor, or premature failure of the capacitor. Failures under these conditions are usually catastrophic and result in hundreds of thousands of dollars in damage to a system and extended off-line periods for repair.
A standard capacitor using dry film technology is the wound capacitor. Wound capacitors are constructed by sandwiching a dielectric film such as polycarbonate, polypropylene, or polyester film between metal electrodes (e.g., vapor deposited metal film). Once formed, the combination dielectric/metal material is wound to form a capacitor. Some specific examples of wound capacitors are found in the following: U.S. Pat. No. 4,719,539 (Lavene), U.S. Pat. No. 4,685,026 (Lavene), and U.S. Pat. No. 5,614,111 (Lavene). Each of these U.S. patents are incorporated herein by reference.
The size of a metallized film capacitor is dictated by the thickness of its dielectric film. The thickness of the dielectric, in turn, is dictated by the required overall breakdown voltage of the capacitor. For instance, if a manufacturer cites a particular film as having a dielectric strength of 200 volts/micron and the capacitor design calls for a dielectric breakdown voltage of 400 volts, then the film may be 2 microns thick. Thus, the breakdown voltage of a capacitor depends on the dielectric strength and the thickness of the film.
When electrical current is passed through a wound film capacitor, thermal energy is generated raising the temperature of the capacitor. In large current applications (for example, 7 amperes to 30 amperes), this thermal energy can be quite large and may lead to the deterioration of the capacitor. In some applications the thermal energy may even lead to an explosion.
It is known to provide sensors to prevent capacitors from overheating or exploding. U.S. Pat. No. 7,471,498 (Laverne) and U.S. Pat. No. 7,471,499 (Bond) disclose a wound capacitor, which includes a hollow core formed by a non-conducting tubular section, and a capacitor winding wrapped around the tubular section. A sensor is disposed within the hollow core. The sensor is configured to sense a predetermined temperature level within the hollow core and provide an alert external to the capacitor winding. Each of these U.S. patents are incorporated herein by reference.
The present invention, as will be described, includes multiple wound film capacitors. These film capacitors are wound individually, one on top of another. For example, a first capacitor is wound on a hollow core. A second capacitor is wound around the first capacitor, in which the latter is used as a winding core. A third capacitor is wound around the second capacitor and so on. As will be explained, these new capacitors provide increased electrical efficiency and are operated in a cooler environment, thereby resulting in higher reliability.